The utilization of carbon dioxide as a useful material rather than a waste product has been of increasing interest in recent years. For example supercritical carbon dioxide (sc-CO2) has been used in place of organic solvents for applications such as dry cleaning of clothes and high pressure liquid chromatography. The displacement of petroleum-based solvents has environmental benefits in itself and these are further enhanced if the CO2 is obtained from a source that would otherwise release it as an atmospheric pollutant.
Another large application for CO2 is as a flooding agent to obtain additional production from spent oil wells. It is estimated that several billion standard cubic feet of CO2 are injected into U.S. oilfields each day with a concomitant recovery of several hundred thousand barrels of oil. It has been known for many years that if the viscosity of the CO2 could be increased, the efficiency of the oil recovery would be greatly enhanced since it would reduce fingering and poor volumetric sweep efficiency. To this end, a great deal of resources have been spent researching the use of surfactants to increase the viscosity (thicken) of high pressure CO2. Carbon dioxide foam floods are typically conducted via the alternate injection of aqueous surfactant solution slugs and pure CO2 (SAG). The idea of adding surfactant to CO2 for performance and/or mobility control was suggested decades ago and to this end there have been recent developments in the identification and design of CO2 soluble surfactants that generate foams.
As is well known to those skilled in the art, oil bearing formations initially contain brine and years of enhanced recovery techniques, including water flooding have added additional fluids. This has necessitated the use of alternating slugs. For surfactants to be effective in downhole environments, the appropriate balance CO2-philicity and hydrophilicity must be maintained. Maintaining this balance has proved difficult for existing surfactant technologies. Furthermore, this type of molecule is not readily soluble in CO2 and must be injected into the formation in the brine phase.
The prior art comprises patents and patent applications issued on polycarbonates as surfactants, which generally teach that there needs to be a hydrophobic section and a hydrophilic section using a standard surfactant model which of course would mean that the surfactant is water soluble and/or miscible. In order to accomplish this result, the surfactant polycarbonates have polyether or epoxide sections and polycarbonate sections and generally specify that below 15 mol % CO2 incorporation, the materials were hydrophilic and above 15 mol % CO2 incorporation, they were hydrophobic. Based on that, if the molecule were greater than 15 mol % CO2 they would not be surfactants since they would possess negligible hydrophilic character.
There remains a need for a CO2-soluble surfactant which albeit hydrophobic in nature, still possesses surface activity, is readily biodegradable and wherein the surface activity allows the CO2-containing surfactant to wet surfaces which prior art CO2-containing surfactants have been unable to do. This ability to wet surfaces is extremely important in EOR operations employing CO2 floods, foams, etc., since it allows the surfactants/CO2 to extract oil from formations and interstices in the formation otherwise not amenable to extraction because of the inability for those surfaces to be adequately wetted.
There also remains a need for a CO2 soluble surfactant which still possesses surface activity, is readily biodegradable and wherein the surface activity allows the CO2 containing surfactant to predictably form foams in porous media for the purpose of controlling the mobility and improving the sweep efficiency of liquid or supercritical CO2 when such CO2 is injected into geological formations for the purpose of storage or sequestration of the CO2.
Nonetheless, there remain challenges with the use of CO2 in these and other applications. In particular, there are very few surfactants that work well with sc-CO2. Those that have worked well, tend to be expensive and/or have drawbacks because of their potential to contaminate the environment or product streams.
Meanwhile, another strategy being explored for the productive use of CO2 has been as a chemical feedstock for the manufacture of chemicals and polymers. In particular, aliphatic polycarbonates (APCs) manufactured by copolymerization of carbon dioxide and epoxides are emerging as promising materials since the polymers have the potential to replace traditional petrochemical polymers. The incorporation of CO2 which accounts for up to 50% of the polymer masshas environmental benefits. CO2-based polymers have been looked at in the past as CO2-soluble surfactants e.g. (WO/2010/062703), but the approach taken has been to append a hydrophilic moiety such as PEG to the polycarbonate which is expected to act as a CO2-phile. To date none of these materials has been found suitable for commercial application.